The present invention relates to data communications and, more particularly, to mobile data communications.
A mobile data communications channel is sometimes referred to as an "impaired channel" since it is affected by a number of channel impairments like Rayleigh fading, co-channel interference, etc., that increase the error rate and, thus, degrade the overall performance of the mobile data connection. One form of mitigating the affects of an impaired channel is to simply apply land-line modem standards to the mobile radio environment.
In the land-line modem arena, International Telephone and Telegraph Consultative Committee (CCITT) standard V.32bis is a representative modulation standard. In V.32bis, during the "start-up mode," or "connection phase," both modems, or endpoints, establish the data connection, e.g., perform a "handshaking" sequence to establish the modulation standard, error control, and the data rate. After the start-up mode, the "data mode," or "communications phase" is entered in which data, or information, is exchanged between the two modems over the mobile data communication channel. One of the features of V.32bis is the ability to sequentially, and automatically, "autorate" upwards or downwards between 4800 bits per second (bps) and 14400 bps during either the start-up mode or the data mode as a function of the error characteristics of the communications channel. For example, if a land-line data connection is initially established between two modems at 14400 bps and the error rate subsequently increases beyond a predetermined threshold, one of the modems will negotiate a data rate change down to the next lowest data rate. In this example, the data rate would change from 14400 bps to 9600 bps. At the latter data rate, information is then transmitted for a period of time to monitor the resulting error rate. If the error rate is still above a predetermined threshold, one of the modems will, again, negotiate a data rate change down to 4800 bps. On the other hand, if the error rate decreases for a period of time while transmitting data at 9600 bps, one of the modems will negotiate a data rate change to the next highest data rate, i.e., from 9600 bps to 14400 bps. In either event, this "error-based" autorate feature takes advantage of the fact that the error rate of a communications channel is typically also a function of the data rate, i.e., the higher the data rate, the likelihood of an error increases.
Thus, in the context of a mobile data communications channel, the land-line autorate feature is used to assist in mitigating the affect of an impaired channel on the communication of information between two modems. If transmitting at 9600 bps and the error rate decreases for a period of time below a predetermined threshold, one of the modems will negotiate a data rate change to 14400 bps in an attempt to improve the data throughput of the mobile data communications channel. Consequently, if the channel condition permits it, a modem will automatically "fallforward," i.e., increase its data rate.
However, in a rapidly changing environment like the cellular environment, a high data rate, e.g., 9600 bps, increases user frustration during "interactive sessions" due to the higher probability of transmission errors. During interactive sessions a user only occasionally sends data--yet any error is immediately seen and felt by a user. For example, a user may send their "password" to the far-end endpoint to "login" to a distant computer facility. Since these occasional data transmissions are subject to a higher probability of error due to the high data rate, a user may experience delays because the modem's error-control routine may have to re-send data.